Engines, such as internal combustion engines and diesel engines, produce drive torque that is transferred to a drivetrain. A forced induction system, such as a turbocharger, can increase the drive torque of the engine without significantly adding weight. By increasing the charge air density in the cylinder, additional fuel can be added and a higher combustion pressure is generated in each cylinder thereby improving the power-to-weight ratio for the engine. In order to achieve this boost in drive torque, a turbocharger converts exhaust gas flow energy to mechanical energy via a turbine. The turbine is connected to an intake air compressor via a shaft. The turbine is positioned in the exhaust flow and the compressor is positioned in the air intake flow.
The turbocharger can operate at a high temperature due to the exhaust gas flowing therethrough. Additionally, the turbine, shaft, compressor, and associated bearings can spin at a high rate of speed, such as up to 100,000 rpm or more. The turbocharger utilizes lubricant, such as engine oil, to lubricate the rotating members and also to cool the turbocharger. The oil is provided by the engine's mechanical oil pump.
The turbocharger can fail when the temperature of the turbocharger bearings is higher than the operating limit for the turbocharger. The turbocharger can also fail when the lubricant flow is insufficient to lubricate the rotating components. The turbocharger can also fail when the lubricant cokes in the turbocharger bearings due to high temperature with low or stagnant lubricant flow.
The drive torque generated by the engine is transferred through a transmission that multiplies the drive torque by a gear ratio. Transmissions generally include multiple gear ratios through which the drive torque is transferred. Automatic transmissions automatically shift between gear ratios based on driver input and vehicle operating conditions. Hybrid powertrains typically include an electric machine and an energy storage device (ESD). In one mode, the electric machine drives the transmission using energy stored in the ESD. In another mode, the electric machine is driven by the engine to charge the ESD.
Traditional transmission control systems determine shift decisions based on vehicle speed and throttle. The shift strategy is developed based on vehicle performance, drivability, and fuel economy based on anticipated driving conditions. The shift strategy also must account for engine sub-systems (e.g., variable valve timing (VVT)) and other features including, but not limited to, powertrain braking, throttle-position, sensor-based shifting, and hybrid vehicle functions. In a hybrid powertrain, the shift control strategy must also account for electrical requirements (i.e., driving or powering the electric machine).
Hybrid powertrains can be operated to improve the efficiency of the powertrain. As such, the electric machine can be activated during advantageous operating conditions to provide supplemental and/or the entire drive torque transferred through the transmission. As a result of this architecture, the engine is frequently turned on and off during normal operation of the vehicle. Additionally, the turning on and off of the engine can be done abruptly. That is, the engine can be shut off immediately when an opportunity to improve the efficiency through the use of the electric machine is presented or at the completion of a charging operation. The abrupt shutoff does not allow the operation of the engine to slowly shut down, such as when coasting (accelerator pedal let off) and/or parking a vehicle wherein the engine is gradually reduced to idle as typically occurs in a non-hybrid vehicle. Additionally, the abrupt shutting off of the engine may result in the accessories being driven by operation of the engine also being turned off abruptly, such as the engine's mechanical oil pump.
The frequent on and off events present a significant challenge in terms of meeting the lubricating and cooling needs of the turbocharger. Of particular concern is the supplying of an adequate lubricant flow to lubricate the turbocharger and provide cooling thereto during periods when the engine is turned off. When the engine is turned off, the engine oil pump is also shut down and no longer supplies oil to the turbocharger. The components of the turbocharger, however, depending on the operating condition immediately prior to engine shutoff, may continue to spin and generate heat as the turbocharger spools down. Hybrid powertrains present additional difficulties as the shutoff of the engine can be abrupt and can occur during a high rpm situation wherein the turbocharger is active and rotating at high rpms. As a result, when a turbocharged engine is utilized in a hybrid powertrain the control strategy must account for the lubricating and cooling needs of the turbocharger when the engine is being frequently turned on/off and when such turning off happens abruptly.